Conformal coatings are widely used in both the military and industrial electronics applications, for protecting circuit board assemblies from moisture, dust, chemicals, and temperature extremes, to prevent damage or failure of the electronic components. While the use of conformal coatings offers several advantages compared to uncoated circuit board assemblies, their application constitutes a “wet-process” which requires the use of hazardous chemicals that must be applied by spraying, brushing, or dipping, followed by drying and/or curing processes.
In addition, it would be difficult to control the conformal coating thickness as well as the formation of pin-holes. With the exception of parylene, which must be applied by expensive vacuum-deposition equipment and which does not lend itself to high-volume production, most organic conformal coatings are readily penetrated by water molecules.
For a conformal coating to be effective, ionizable contaminants, such as salts, must be prevented from reaching the circuit nodes where they can combine with water to form microscopically thin electrolyte layers that can be both corrosive and electrically conductive. Also, for the conformal coatings to adhere properly to the circuit board assemblies, thereby minimizing peeling, de-wetting, and the propensity to form pin-holes, all surface contamination must be removed prior to the application of the conformal coating, using another “wet-process” such as a vapor degreasing or semi-aqueous washing in a special equipment. Special shielding and masking measures must also be taken while applying the conformal coatings to prevent it from contaminating connectors, sensitive components and the circuit board assemblies.
The application of a close-fitting, thin-layer of polymer, or another material in flat-sheet form, over the circuit board assembly and its electronic components, either by a vacuum or pressure molding, or by other suitable processes, would offer superior protection from moisture, dust, chemicals, and temperature extremes compared to conformal coatings. A thin polymer layer, or multiple thin layers, could be selected to provide various additional attributes, such as, improved heat dissipation, ESD and EMI protection and control, and protection from handling and in-use shocks.
Thin polymer layers could be added to the circuit board assemblies for use in non-potted as well as potted applications. In potted applications, the polymer layers would offer additional benefits such as forming a barrier to prevent the potting material from seeping into areas around and underneath sensitive components. After being cured, potting materials could cause high stresses, such as a residual stress and a thermal expansion stress, during temperature cycling, due to the coefficient of thermal expansion mismatches and also due to contraction and expansion of the potting material itself.
More specifically, potting materials are being used with increasing frequency, in both commercial and military applications, to encapsulate the electronic components and circuit board assemblies of electronic systems. The use of potting materials allows for a simpler support-structure (while also enabling a smaller over-all system design) as well as enhanced structural support for the electronic components and circuit board assemblies against shock and vibration.
A major disadvantage with encapsulants or potting materials however, is the fact that they are permanent solid bodies that prevent any access or servicing of the components they encapsulate. Potting materials are almost always thermoset materials that harden once and cannot not be re-softened or reused
In numerous military munition designs, where the electronic components must survive the extremely high g-forces experienced during gun-launch, the potted electronics are inactive until the munition is used. Until this time the munition may have been in storage without environmental (temperature and humidity) controls for up to 20 years.
In contrast, the electronics for most commercial applications tend to be active for most of their lifetime where the operating environment is more stable and predictable. Without external temperature controls, or the fairly constant temperature environment that active electronics create for themselves, inactive electronic components experience continuously varying physical stresses which are created due to their intimate contact with the potting material and the different rates of expansion and contraction that each produces with changes in temperature. If the changes in temperature are severe enough, or repeated a sufficient number of times, the physical stresses induced on the inactive electronic components can be severe. The resultant loads or stresses can be high enough to fracture the ceramic lids of hollow-cavity devices, or other types of electronic components, and may also lift components completely off of their circuit boards.
In addition, during the potting process, the potting material may seep into the open spaces between the leads of the chips and also underneath the chip packaging. The potting material that has seeped into these areas will create residual stresses in the solder joints and also against the packaging bottom surface after the potting material has solidified during the curing process.
Currently, the following failures have been observed for potted electronics during either the temperature-cycling qualification process or the life test (temperature-cycling and gun-launch) of a sub-system of the fielded-artillery system:                1. Solder joints failed during the temperature-cycling process.        2. Solder joints failed during the life test.        3. Lids and lid-seals cracked on MEMS (e.g. MEMS—Micro-Electro-Mechanical Systems) open-cavity devices during the temperature-cycling process and also during the life test.        4. Tiny electronic devices pulled off from their circuit boards during the temperature-cycling process.        
The application of a barrier, such as a thin layer of polymer (or other material), over the electronic components prior to the addition of the potting material, is believed necessary to help mitigate the above failures. The polymer layer can be applied by various processes such as heat, vacuum, vacuum plug assist, radio frequency forming or a combination of processes, or a combination of the above.
For failures resulting from the solder joints failure and components being pulled off their circuit board assemblies during temperature cycling, the polymer layer would prevent the potting material from intruding between the chip-leads and also under the chips, and thus help prevent the push and pull stresses that the potting material would produce as it expands and contracts with increasing/decreasing temperatures.
For failure resulting from both lids and lid-seals being cracked on hollow-cavity devices, the polymer layer would provide: 1) a low-adhesion boundary between the potting material and the lid surfaces thereby mitigating the high shear-stresses that would develop as the potting material expands and contracts due to ambient temperature fluctuation, and 2) a compliant layer that would minimize the high-compression stresses that the potting material develops when it expands, due to increasing temperatures, and high-tension stresses that the potting material develops when it shrinks, due to decreasing temperatures, against the lids of these devices.
What is therefore needed is a process of forming and emplacing the thin polymer, or other formable composite, layers so as to precisely conform to the imprecise geometries of the electronic components on the circuit board assemblies, despite the imprecise geometries of these components due to their geometrical tolerance, placement tolerance as well as the manufacturing and assembly variances of the circuit board assemblies. It would also be substantially advantageous that the polymer layers be sufficiently strong to provide the structural support of the potting to the circuit board assemblies during high-g force events. It would further be desirable to have the thin polymer layers be sufficiently flexible to allow for differentials in coefficients of thermal expansion between the circuit board assemblies and the potting material.
Certain publications, such as U.S. Pat. No. 5,318,855, propose a method to vacuum form a polymer film over the circuit board assemblies to provide electrical and environmental protection. However, the proposed method does not seem to allow the polymer layer to precisely conform to the electronic components.
U.S. Pat. Nos. 4,959,752 and 4,768,286 suggest vacuum forming polymer layers over a circuit board assembly to closely conform to the geometry of the circuit board assembly prior to the application of the potting material. However, these layers must be thin enough to permit vacuum forming over the circuit board assemblies, and are therefore too thin to provide sufficient structural support or to provide sufficient boundary to differential thermal expansion.
Another process of forming multiple layer films into packages to protect printed circuit boards is described in U.S. Pat. No. 7,161,092. This patent generally describes a method of forming a plurality of layers to cover the approximate shape of a printed circuit board assembly as opposed to mounting the electronic components in a container or enclosure. This method describes the bonding of at least three layers (i.e., insulating, conductive, and abrasion protection) into a conformal film that can be stamped or pressed, and then adhered to the electronic component assemblies, which may require breather valves. The surface tension of the individual layers usually provides an approximate fit, that is a fit with substantial radii or a loose fit encapsulating technique.
What is therefore needed is a process of forming the layers without the need for bonding individual layers together, and that produces a very tight fit between the polymer layers and the circuit board assembly. In addition, desirable process would not require adhesives or melting operations to join the layers to the circuit board assembly, and would provide an exacting polymer layer fit that facilitates snapping of the polymer layer to the assembly it is designed to protect. Such a desirable process would provide a polymer layer that follows the contours (or profiles) of all the electronic components on the circuit board assembly and would allow for variances in dimensional tolerances, placement and assembly. Prior to the advent of the present invention, the need for such a protective layering process has heretofore remained unsatisfied.